The invention relates to a process for preparing non-porous, selective membrane layers. Various processes for preparing membrane layers are known, such as phase-inversion solvent spreading, dipping, spraying and plasma treating. The former processes have been found useful to prepare membrane layers which can be highly permeable for certain fluids, however, in many cases at the expense of the selectivity (i.e. the ratio of permeabilities of at least two fluids in a mixture). The latter process i.e. plasma treating of a substrate with an inert gas, which may be combined with a polymerizable hydrocarbonaceous compound, is known to be suitable for the preparation of non-porous (i.e. dense) selective top layers of composite multi-layer membranes. However, the selectivities attained with membranes thus prepared are in some cases not sufficiently high for commercial application in e.g. gas separation processes.